In electrophotography, a latent image is created on the surface of a photoconducting material by selectively exposing areas of the charged surface to light. A difference in electrostatic charge density is created between the areas on the surface exposed and unexposed to light. The visible image is developed by electrostatic toners containing pigment components and thermoplastic components. The toners are selectively attracted to the photoconductor surface either exposed or unexposed to light, depending on the relative electrostatic charges of the photoconductor surface, development electrode and the toner. The photoconductor may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles. For laser printers, the preferred embodiment is that the photoconductor and toner have the same polarity, but different levels of charge.
A sheet of paper or intermediate transfer medium is then given an electrostatic charge opposite that of the toner and passed close to the photoconductor surface, pulling the toner from the photoconductor surface onto the paper or intermediate medium, still in the pattern of the image developed from the photoconductor surface. A set of fuser rollers fixes the toner to the paper, subsequent to direct transfer, or indirect transfer when using an intermediate transfer medium, producing the printed image.
The toner may be in the form of a dust, i.e., powder, or a pigment in a resinous carrier, i.e., toner, as described, for examples in Giaimo, U.S. Pat. No. 2,786,440, issued Mar. 26, 1957. The toner particles may be used or fixed to the surface by known means such as heat or solvent vapor, or they may be transferred to another surface to which they may similarly be fixed, to produce a permanent reproduction of the original radiation pattern.
Dry development systems suffer from the disadvantage that distribution of the powder on the surface of the photoconductor, and the charge to mass ratio of the particles, are difficult to control. They can have the further disadvantages that excessive amounts of dust may be generated and that high resolution is difficult to attain due to the generally relatively large size of the powder particles, generally greater than 5 .mu.m. When particle size is reduced below 5 .mu.m, particle location becomes more difficult to control. Many of these disadvantages are avoided by the use of a liquid developer of the type described, for example, in Metcalfe et al., U.S. Pat. No. 2,907,674, issued Oct. 6, 1959. Such developers usually comprise a non-polar and non-conducting liquid which serves as a carrier and which contains a dispersion of charged particles comprising a pigment such as carbon black, generally associated with a resinous binder such as, for example, an alkyd resin. A charge control agent is often included in order to stabilize the magnitude and polarity of the charge on the dispersed particles. In some cases, the binder itself serves as a charge control agent, also known as a charge director.
Liquid developers are also frequently used in toner transfer systems. When so used, they must give consistently high uniform density not only on the element on which the image is initially formed but also on the transfer or receiver sheet.
It is necessary in electrophotography to have electrical charge on the toner particles in order to impel them to move toward the photoconductor surface via electrical field. The principle is easily achieved in dry powder systems, but more difficult in liquid toners. The reason for the difficulty is that the solution phase for liquid toners makes it impossible to charge the particles triboelectrically. Instead, they must have formal and relatively permanent charge arising either from their chemistry, or from non-specifically adsorbed species which are themselves permanently charged. In addition, the charge on the particles must not cause flocculation or destabilization of the toner, and must remain on the particles, keeping the bulk conductivity of the solution phase at a low and controlled level.
Several patents teach methods of charge direction for liquid electrographic toners. One method, disclosed in U.S. Pat. No. 4,925,766 (Elmasry et al.), shows the use of metal soaps (such as Z.sup.4+ soap) to provide metal ions (such as Zr.sup.4+ ion) which are then more or less bound coordinatively on the resin coating of the pigment. Several functional groups may be incorporated into the resin to provide the binding sites for the Zr.sup.4+ or other metal. These binding functionalities are shown in cols. 9 and 10 of this patent. They typically possess oxygen or nitrogen, or occasionally sulfur, to donate electron pairs into the coordination sphere of the metal ion. The oxygen donor sites are typically protonated, such as in carboxylic acids and phenols. Alternatively, they may be non-protonated, such as in nitrogen donor atoms or beta-diketones. These electron donor groups are ideally bi- or poly-dentate so as to chelate, i.e., bind, to the metal atom at two or more points. The advantage of chelating and other polydentate ligands, as opposed to monodentate ligands, is that they increase the probability that the metal ion will actually be located on the toner particle, and not associated with the liquid phase. When the charged metal species is unbound, and in the liquid phase, it contributes to bulk phase conductivity of the medium, and not to migration of the toner particle in the field. In fact, it even suppresses toner migration due to its greater electrophoretic mobility.
Another disadvantage of these metal soap charge direction systems is that many of them, and the most widely used ones, employ protonated binding sites. This means that when the metal is bound into the resin the proton with its associated charge must go somewhere. If it goes into the continuous phase it contributes to background conductivity and serves to suppress particle migration in the electrical field. There is residual water in virtually all liquid toners, and the proton may go into the residual water. If this happens there may be micro-micellar formation which can promote flocculation of the toner. This is one possible explanation for the observed flocculation phenomena in this type of toner.
Another patent U.S. Pat. No. 5,045,425 (Swidler) teaches incorporation of salicylates in the resin, and addition of Al.sup.3+ complexes of salicylates to the dispersion. In this case, the formation constant of the Al.sup.3+ complex with the surface salicylate groups is high, and if the total concentration of the aluminum is optimized, most of it is bound to the surface of the toner particle. The remainder of the aluminum is bound up in homogeneously dispersed complexes, in the liquid phase. The role of these complexes in overall measured conductivity of the toner is unclear, but certainly does nothing to promote migration of the toner particles toward the discharged areas of the photoconductor.
The article "Mechanism of Electric Charging of Toner Particles in Nonaqueous Liquid with Carboxylic Acid Charge Additives" by K. Pearlstine, L. Page and L. El-Sayed, Journal of Imaging Science, Vol. 35, No. 1, Jan./Feb. 1991, pp. 55-58, discloses toner particles containing carboxylic acids substituted with electron-withdrawing groups as charge directors. The carboxylic acid groups disclosed in this article are bound, or associated with, the toner particles by Van der Waals forces.
In these prior art cases then, a metal ion more or less bound to the particle surface is used as the charge director. The resulting charge on the particle is thus more or less semi-permanent and electrically positive. There is, however, a high probability that at least some of the total charge in the system is spread uniformly throughout the continuous phase and not localized on the particles.
Other prior art toner systems exist which rely on the non-specific adsorption of a large, negatively charged organic species such as lecithin to provide negative charge direction. See, for example, U.S. Pat. No. 4,897,332 (Gibson et al.). There are two main disadvantages of these systems. First, the charge is not bound to the particle as a permanent or semi-permanent part of the structure, but is rather loosely associated with it, via Van der Waals forces. Secondly, in order to achieve significant charge on the particles, it is necessary to add excess charge director material to the liquid toner. This invariably means there will be an excess of unassociated charge director in the continuous phase which, as before, actually suppresses the desired migration of the toner particles in the field.